Ferro-Electric Device And Modulatable Injection Barrier

ABSTRACT

Described is a modulatable injection barrier and a semiconductor element comprising same. More particularly, the invention relates to a two-terminal, non-volatile programmable resistor. Such a resistor can be applied in non-volatile memory devices, and as an active switch e.g. in displays. The device comprises, in between electrode layers, a storage layer comprising a blend of a ferro-electric material and a semiconductor material. Preferably both materials in the blend are polymers.

FIELD OF THE INVENTION

The invention relates to a to a modulatable injection barrier and to asemiconductor element comprising same. More particularly, the inventionrelates to a two-terminal, non-volatile programmable resistor. Such aresistor can be applied in non-volatile memory devices, and as an activeswitch e.g. in displays. The invention also relates to programmable andrectifying diodes.

BACKGROUND TO THE INVENTION

Ferro-electric devices are employed as part of an electronic (e.g.computer) memory. Their function is based on an electronicallyprogrammed binary state (a “0” or a “1”), which can be read outelectronically as well. A plurality of such devices is used to form amemory. The programming requires a voltage to polarize theferro-electric material.

Non-volatile memory devices have the desired property that they will notloose their programmed state off-power. Conventional, non-volatilememory devices are not, however, intrinsically non-volatile. In thecontext of the invention, this means that the voltage needed forread-out will not change the polarized state of the device. I.e., eachread-out of a device which is not intrinsically non-volatile, will haveto be followed by step in which the device is polarized back again intothe original state. This is a drawback, particularly in view of thelimits the repeated poling puts on the life of the device.

A reference in this regard is a paper by Blom et al., Physical ReviewLetters, Volume 73, Number 15, 10 Oct. 1994, 2109, which describes aSchottky diode. More precisely, Blom describes a two-terminal memorydevice comprising

-   a first electrode;-   a second electrode; and, adjacent to said electrodes,-   a film of a ferro-electric semiconductor material, viz. PbTiO₃.

In this device, the binary states are determined by a high or a lowconductance state of the ferro-electric semiconductor. This is neitherpractical, nor workable, as the conducting properties go at cost of theferro-electric properties, i.e. this leads to a device which eithercannot be polarized or cannot be read-out.

It is an object of the invention to provide an electrical switch whichis intrinsically non-volatile. The high and low conductance state serveas the binary ‘1’ or ‘0’ (or vice versa) in a memory application, or asan active electrical switch to select, e.g. a display pixel. Furtherobjectives are to increase storage density, to decrease the processingtemperatures such that the fabrication of the non-volatile electricalswitch is compatible with back-end silicon, to reduce the processingcosts, and to realize an a-symmetrically switching diode (that is alwaysin reverse in one bias direction).

Moreover, it is an object of the invention to provide a modulatableinjection barrier (in effect mostly a modulatable electrode) that can beused not only in memory devices and switches, but also in other devicessuch as three terminal (field-effect) devices, and rectifying orlight-emitting semiconductor diodes. More particularly, it is an objectof the invention to provide organic semiconductor diodes which allowmore than one programmed state (i.e. creating an asymmetricallyswitching diode).

Another object of the invention is to provide memory devices, notably ifbased on organic materials, which are capable of avoiding crosstalk.

As a further background on the use of organic semiconductor materials,the following references are referred to Cheng Huang et al., inElectrets 2005, pages 91-94, provides a nanocomposite having a highdielectric constant. Herein a high dielectric matrix polymer is providedwith polyaniline conductive filler. With reference to the conductivefiller concentration being close to the percolation concentration, theaim is to obtain a material of high dielectric constant that isparticularly useful for its electromechanical response. The referencetherewith does not relate to a modulatable injection barrier, as the aimof the document goes against obtaining a state of conductivity. Similarconsiderations hold for a reference by the same first author, in AppliedPhysics Letters, 87, 182901 (2005), pages 182901-1-1982901-3.

Background references on memory elements using ferro-electric polymersare WO 2006/045764 and WO 02/43071.

SUMMARY OF THE INVENTION

With the aim to meet one or more of the aforementioned objectives, theinvention provides, in one aspect, a semiconductor element comprising atleast one modulatable injection barrier, said barrier being formedbetween at least one electrode layer and a semiconductor layer, whereinthe semiconductor layer comprises a blend of a semiconductor materialand a ferro-electric material. The semiconductor element can be a deviceby itself, or can be used in semiconducting devices.

In a further aspect of the invention, the modulatable injection barrierserves, in various thinkable electronic devices, to program a memorydevice, to select a switch position, or to create a rectifying diode.

In another aspect, the invention provides a two-terminal non-volatileprogrammable resistor (to be used in e.g. electronic memory devices orin selection devices such as switches) comprising, preferably on asubstrate:

-   (a) a first electrode layer;-   (b) a second electrode layer; and, in contact with said electrode    layers,-   (c) a storage layer comprising-   (d) a ferro-electric material,    which storage layer separates the first and second electrode layers    from each other, i.e. sandwiched between the electrodes, wherein the    storage layer comprises a blend of the ferro-electric material (d)    with-   (e) a semiconductor material.

The blend thus comprises a ferro-electric phase and a semiconductorphase. The invention allows both the semiconducting properties and theferro-electric properties to be retained and optimised in one and thesame device.

The device can be switched from one binary state to the other throughpolarization of the ferro-electric, using a relatively high firstvoltage V_(p). As a result of the semiconducting phase, a relatively lowsecond voltage V_(r) can be applied to read-out the device. As V_(r) isinsufficient to re-polarize or de-polarize the ferro-electric, thedevice is intrinsically non-volatile.

In another aspect, the invention provides a two-terminal electricalswitch comprising a storage layer sandwiched between first and secondelectrode layers, wherein the storage layer is a polymer layercomprising a blend of a ferro-electric polymer and a semiconductorpolymer.

In another aspect, the invention provides a three-terminal device,comprising a blend of ferro-electric polymer and a semiconductingpolymer as active layer on top of an insulating layer. The insulator issandwiched between a gate electrode and the active layer, to which asource- and drain electrode are applied. By polarizing theferro-electric via the gate electrode the injection barrier of eitherthe source- or drain electrode or both is modulated, leading to a threeterminal non-volatile memory device or a three terminal switch withimproved switching characteristics in terms of on/off ratio andsub-threshold slope.

In another aspect, the invention provides a two-terminal memory deviceas described above, comprising a blend of a ferro-electric polymer and asemiconductor polymer and two asymmetrically switching electrodes.

In yet another aspect, the invention provides a memory cell comprising aplurality of memory devices having a storage layer as described abovewith reference to a blend of a ferro-electric material and asemiconductor material. Also in other devices, i.e. not only inmemories, but also in e.g. light emitting diode arrays or displays, aplurality of semiconductor elements according to the invention can beapplied.

In yet another aspect, the invention provides an electrical switch cellcomprising a plurality of electrical switches having a storage layer asdescribed above with reference to a blend of a ferro-electric materialand a semiconductor material

DETAILED DESCRIPTION OF THE INVENTION

The various elements of the present invention are discussed hereinbelow.

In a general sense, the invention provides a modulatable injectionbarrier formed between an electrode layer and a semiconductor layer. Theterm “injection barrier” is known to the skilled person, and relates tothe energy step or energy barrier that carriers (electrons or holes)have to overcome when being injected from an electrode into asemiconductor material at their interface.

The term “modulatable” refers to an injection barrier of which therequired energy step for charge injection can be switched from a low toa high level. For a high energy barrier only few charge carriers can beinjected into the semiconductor (low-conducting state), whereas for alow barrier a large amount of carriers can be injected, leading to alarge current (conductive state).

To provide the modulatable injection barrier, according to theinvention, the semiconductor layer comprises a blend of a semiconductormaterial and a ferro-electric material. Without theory to be consideredas binding, it is believed that poling of the ferro-electric material,when in contact with the electrode, will allow carrier injection intothe semiconductor material, in other words, will allow current to flowinto the semiconductor material, in direct or indirect contact with thesame electrode. The blend of the semiconductor material and theferro-electric material thus serves as a storage layer preserving(unless and until reversed) either of two biases. The nature of thestorage layer will be discussed further below.

This can also be applied vice versa, i.e. starting from a situationwhere a semiconductor has conducting properties, modulate the poling ofthe ferro-electric material so as to create a barrier for carriers to beinjected into the semiconductor. The modulatable injection barrier ofthe invention thus allows, e.g., a “good” contact (i.e. anelectrode-semiconductor interphase at which current readily flows) to betransformed into a “bad” contact (i.e. an interphase at whichcurrent-flow is low or zero), or vice versa. The skilled reader willunderstand that variants between “good” and “bad” contacts are possible.

The modulatable injection barrier is particularly useful in organic orpolymeric light-emitting diodes (OLEDs or PLEDs). These diodes on oneside have an electron-injecting contact, which, to be efficient, isusually made of reactive materials such as barium or calcium, whichgenerally is adverse to the stability of the LED. By virtue of theinvention, a non-reactive contact can be used, e.g. aluminum, which byitself is not a good injector of electrons. In conjunction with theferro-electric, as foreseen in the invention, the electron injectionfrom the non-reactive electrode is brough to a much higher level, as anon-reactive replacement of electrodes made of such reactive materialsas calcium and barium.

The modulatable injection barrier of the invention also serves, e.g., toprogram a memory device or to select a switch position in a selectiondevice. Particularly for the latter two devices, the invention, in oneembodiment, can be described as a two-terminal non-volatile electricalswitch comprising (a) a first electrode layer; (b) a second electrodelayer; and, in contact with said electrode layers, (c) a storage layercomprising (d) a ferro-electric material, which storage layer separatesthe first and second electrode layers from each other, wherein thestorage layer comprises a blend of the ferro-electric material (d) with(e) a semiconductor material.

The electrodes (a) and (b) are regular components of memory devices,switches, transitors, light-emitting diodes and the like. The sameelectrode materials can be used which are well-known for semiconductordevices such as transistors, diodes, et cetera. Suitable materialsinclude tungsten, silver, copper, titanium, chromium, cobalt, tantalum,germanium, gold, aluminum, magnesium, manganese, indium, iron, nickel,palladium, platinum, zinc, alloys of the foregoing metals, aluminum,lithium, sodium, barium, calcium, lithiumfluoride, indium-tin oxide,other conductive and semi-conductive metal oxides, nitrides andsilicides, polysilicon, doped amorphous silicon, and various metalcomposition alloys. Also, doped or undoped conducting or semi-conductingpolymers, oligomers, and monomers can be used for the electrodes, suchas poly(3,4-ethylenedioxy thiophene): poly(styrene sulphonate)(PEDOT:PSS), polyaniline, polythiothene, polypyrrole, and derivativesthereof. Electrodes can comprise one or more layers of differentmaterials, or blends from different materials.

The electrodes (a) and (b) can be identical leading to symmetricaldevices or dissimilar yielding asymetric current transport

Preferably, the electrodes are made of metals that do not form Ohmiccontacts with the semiconductor. These enhance the switchingfunctionality of the devices.

The electrodes are applied in a practical order, preferably as mostlogically going with building up the device on a substrate. Thus, thefirst electrode layer (a) is applied on a substrate, e.g. byevaporation. As a next layer, the storage layer is applied comprising ablend as described above. In making the device, a practical advantage isobtained by choosing polymer materials for the blend, as these can beapplied with relative ease, and in desired thicknesses, by techniquesknown in the art of organic and polymer devices, such as spin-coating orprinting.

Before further processing, the storage layer may or may not desire, orrequire, a further treatment, e.g. annealing a polymer blend as knownfor ferro-electric polymers. The ferro-electric properties are enhancedwhen these polymers are crystallized above the Curie temperature.

In the specific embodiment described, with a substrate on which thefirst electrode layer (a) and the storage layer (c) are consecutivelyapplied, on top of the storage layer the second electrode (b) isapplied, which can be done in the same fashion as discussed for thefirst electrode layer (a).

While the devices according to the invention can be built-up eachindividually, it is preferred to create the layer structure of theinvention for a plurality of devices simultaneously. To this end eitheror both of the electrode layers can be provided with shapes commensuratewith any desired circuitry in which the memory devices of the inventionare used. Preferably, the device is built up so as to have crossing barsof electrodes. In order to prevent cross-talk between the devices it isimportant that in both the high and low conductance state the current inreverse bias is low. It should be noted that the invention provides agreat advantage over previous organic memory devices, as these generallyare not rectifying, i.e. prone to cross-talk by allowing current to flowin either direction. The memory devices of the present invention, alsowhen based on organic materials, can be set by poling theferro-electric, to allow current-flow in one direction only.

The storage layer can be a single layer comprising the blendedsemiconductor and ferro-electric phases. It can also comprise severalsublayers of the same blend, or several sublayers of different blends.

The storage layer typically has a thickness of 50-500 nm and preferablyabout 100 nm.

The storage layer comprises a blend of a ferro-electric material and asemiconductor material. Although this blend can be made of inorganicmaterials, e.g. by co-evaporation, it is much preferred to use organicmaterials.

Suitable inorganic ferro-electric materials include PbTiO3, BaTiO3.Suitable inorganic semiconductor materials include silicon, gallium,arsenide.

Preferably, the ferro-electric material and the semiconductor materialare organic materials. Most preferably, the organic materials arepolymeric materials.

Suitable organic ferro-electric materials are nylons and most preferablypoly vinylidene fluoride co polymer with trifluoroethylene(P(VDF-TrFE)). Materials can be either high- or low molecular weight aslong as they are ferroelectric. Also electrets can be used in stead offerro-electric materials because their polarization can be switchedusing an applied electric field.

Suitable semiconductor materials are organic materials such asfullerenes, pyrilenes, phthalocyanines, oligomers of thiophenes,phenylenes and phenylenes vinylenes. Suitable semi-conductor polymers,as preferred, are poly(3-alkylthiophene)s, poly(dialkoxy phenylenevinylene)s, poly(aniline)s, poly(thiophenes), poly(phenylene)s,poly(phenylene ethylene)s, poly(pyrole)s, poly(furna)s,poly(acetylene)s, poly(arylenmethine)s, poly(isothianaphthene)s,poly(fluoren)s, and most preferably region-irregular poly3-hexyl-thiophene rir-P3HT. As an alternative, also solution processableinorganic semiconducting nanoparticels as ZnO, TiO2, CdS, CdSe etc. canbe used in a blend with a soluble ferroelectric layer.

The preferred polymers have the respective structures of formulae I andII below.

In another embodiment, a copolymer is used to provide both of theproperties needed in the storage layer, viz. a block copolymer having asemiconducting block and a ferro-electric block. Preferably such aco-polymer is crystallized with both blocks neatly alternating, so as toprovide an optimal blend according to the invention.

It should be noted that the term “blend” has a broad meaning, indicatingtrue blends of polymers, copolymers, or interpenetrating polymernetworks (IPNs) of such a type as to still comprise separate phases ofthe semiconductor polymer and the ferro-electric polymer.

In the blend, the ferro-electric polymer is present in at least asufficient extent to allow that a polarization charge can be measured(i.e. otherwise the material would no longer be a ferro-electric). Thesemiconductor polymer is present in an amount at least sufficient toallow a path through the blend for travel of a charge carrier betweenthe electrodes.

More particularly, the ferro-electric material (d) and the semiconductormaterial (e) can be blended in a ratio, by weight, of (d): (e) rangingfrom 1:1 to 1000:1, preferably of from 10:1 to 100:1. In order toprevent some semiconductor polymers, such as rir-P3HT, to formaggregates or to macrophase separate, it is preferred of these polymersare present in not too high an amount, preferably calling for ratios of(d):(e) of from 20:1 to 50:1, more preferably of from 20:1 to 40:1.

It is also possible to provide, in the storage layer, a continuous firstphase of an organic material having either of the semiconductor orferro-electric properties, and contained in this, the second phase of aninorganic material having the other of the semiconductor orferro-electric properties. As an example a ferro-electric can beprepatterned with nanometer sized holes that are filled with an(in)organic semiconductor.

The processing is facilitated using a common solvent for bothferro-electric and semiconductor to obtain an intimate morphology.Furthermore, addition of compatibilizers might be helpful as commonlyused in blends of two polymers.

In order to change the switching characteristics additionalsemiconductor layers can be added between the active (blend) layer andelectrodes

The devices of the invention can be advantageously used in memory cells.As is practically preferable, the invention thus also relates to aplurality of injection barriers and devices.

It is to be understood that the invention is not limited to theembodiments and formulae as described hereinbefore. It is also to beunderstood that in the claims the word “comprising” does not excludeother elements or steps. Where an indefinite or definite article is usedwhen referring to a singular noun e.g. “a” or “an”, “the”, this includesa plural of that noun unless something else is specifically stated.

The invention will be illustrated with reference to the following,unlimitative Examples and the accompanying Figures.

FIGURES

FIG. 0 shows a band diagram associated with a non-Ohmic contact(Fermi-level not aligned with the valence- or conduction band) and anOhmic contact (Fermi-level aligned).

FIG. 1 a shows a cross-section of a part of an injection barrieraccording to the invention. In white, it depicts part of a blend ofsemiconductor material and ferro-electric material (indicated withwords). In grey it depicts part of an electrode. As is shown, a polingof the ferro-electric material results at the boundary of theferro-electric material and the electrode in opposite charges (depictedas a negative charge (−) at the ferro-electric side and a positivecharge (+) at the electrode side). The arrows pointing from theelectrode into the direction of the semiconductor material show (as alsoindicated in words) the injection of charge into the semiconductormaterial. The figure also contains dashed lines, to indicate the sourcefor band diagrams as depicted in FIG. 1 b and 1 c. FIGS. 1 b and 1 cshow band diagrams as will be valid for the positions of the dashedlines drawn in FIG. 1 a. The horizontal lines stand for the valence band(low) and the conduction band (high), of the ferro-electric on the left(with the larger band gap) and the semiconductor on the right (with thesmaller band gap), grey being the electrode.

The band diagram shown in FIG. 1 b is valid for the unpoled state of theferro-electric. The arrows from the electrode into the direction of theferro-electric, display the inadequacy of carriers to be injected intothe semiconductor material (indicated as “poor injection”).

The band diagram shown in FIG. 1 c is valid for the poled state of theferro-electric, i.e. the situation shown in FIG. 1 a. The poling of theferro-electric results in a sufficient counter-charge at the electrodeso as to surmount the energy barrier with the semiconductor material,allowing charge carriers to flow into same (indicated as “efficientinjection”).

FIG. 2. shows (a) Ferroelectric hysteresis loop of P(VDF-TrFE) polymer,sandwiched between two silver electrodes, using a Sawyer-Tower technique(b) C-V measurements of the devices with different blending ratio.

FIG. 3. shows IV characteristics of a devices with 1 mm² area. (a)pristine devices (before poling) with different blending ratios varyingfrom device with only P(VDF-TrFE) to 1:20 rir-P3HT:P(VDF-TrFE) and (b)IV sweep for the 1:20 device, showing the hysteresis in current at twodifferent operating states. (c) comparison of the current for apristine, positively poled and negatively poled device with 1:20 blend.

FIG. 4. shows current scaling with area; current density for both ON andOFF states lie on top of each other for two different device areas.

FIG. 5. shows programming time measurement to bring the device from OFFto ON state and vice versa with a ±30 V signal with different width.

FIG. 6. shows data retention time.

FIG. 7. shows IV characteristics of a blend where the rir-P3HT isreplaced by Ni-dithielene. The blend consists of P(VDF-TrFE) 65%-35%with 5% of the Ni-complex, sandwiched between two Ag electrodes. Thechemical structure of Ni-dithielene is also shown.

FIG. 8. shows the I-V characteristics of a blend 1:40rir-P3HT:P(VDF-TrFE) with asymmetric electrodes. As a bottom electrodeAu is used, and as a top electrode LiF/Al is used. Upon poling theinjection of holes from the Au into rir-P3HT is modified (negativebias), whereas the injection of holes from the LiF/Al is always blocked.

EXAMPLES Example 1

On a cleaned glass substrate, first 1 nm of chromium followed by 20-50nm of silver (or gold) is evaporated. Then the solution is spin coatedand a thin film of the storage medium is formed. The stack is annealedfor 2 hours at 140° C. in a vacuum oven in order to enhance theferroelectric phase of the P(VDF-TrFE) subphase. Then, a metal electrodeis evaporated on top of the film using a shadow mask.

Solutions with 50 mg/ml concentration of only P(VDF-TrFE) intetrahydrofuran (THF), 1:100 rir-P3HT/P(VDF-TrFE), i.e. 1 mg rir-P3HTadded to 100 mg P(VDF-TrFE) dissolved in 2 ml THF, as well as 1:50,1:40, 1:30 and 1:20, and 1:10 are made.

Results

First the properties of a device with only the ferroelectric(P(VDF-TrFE) (65-35) sandwiched between two silver electrodes arecharacterized using a Sawyer-Tower circuit. FIG. 2( a) shows the typicalhysteresis loop for this ferroelectric polymer at different voltages ata frequency of 100 Hz. The coercive field is about 50 MV/m and theremnant polarization is around 60 mC/m².

Having characterized the ferroelectric properties of the pureferroelectric material as a next step devices fabricated with differentblends ratio's of rir-P3HT and P(VDF-TrFE) are investigated. Two mainissues are addressed: ferroelectricity and the device conductivity dueto presence of rir-P3HT.

FIG. 2( b) shows the capacitance of the devices from CV measurement, fordifferent blending compositions. Silver and gold are used as bottom andtop contacts, respectively. The device with only P(VDF-TrFE) shows theexpected butterfly-shape CV for a ferroelectric material. When addingthe semiconducting rir-P3HT two features are observed; First thecapacitance decreases with increasing rir-P3HT content in the film.Secondly, the switching voltage increases as compared to a pureP(VDF-TrFE) device. Since all the blend devices show nearly the same CVbehavior as the pristine ferro-electric device it can be concluded fromFIG. 2( b) that in the blend the ferro-electricity is still present inall blends, even in the case of 1:10.

Having demonstrated the presence of ferroelectricity in the blend filmsthe current transport through the devices is investigated. FIG. 3( a)shows the current for devices with differentferro-electric:semiconductor ratio with both silver bottom and topelectrodes before poling (pristine current). As expected, the currentscales with the amount of rir-P3HT in the film, increasing from theleakage current of the P(VDF-TrFE) only device towards a maximum of 10μA for the 1:10 rir-P3HT:P(VDF-TrFE) device. Since the structure of thedevice in this case is symmetric (Ag/(rir-P3HT:P(VDF-TrFE))/Ag), beinginjection limited on both sides, a symmetric IV curve is expected. Thisis only valid for unpoled pristine devices. In a poled device, the IVcurve as is shown in FIG. 3( b) becomes asymmetric due to theferroelectric polarization.

Switching Mechanism:

In order to inject charges into a semiconductor an Ohmic contact isrequired of which the Fermi-level aligns with the valence- orconduction. When the fermi-level is not aligned, the injection of chargecarriers is inefficient. Then the current in the device is low andlimited by the charge injection process. (FIG. 0). In the presentinvention the ferro-electric polarization charge is used to enhance thecharge injection into the semiconductor of an injection limited device.In FIG. 1 a the blend of a ferro-electric material and a semiconductoris schematically represented. Semiconducting percolated pathways of thesemiconductor exist to transport the injected charges to the otherelectrode. For an unpoled ferro-electric the electrode is not capable ofefficient charge injection into the semiconductor. However, when theferro-electricum is poled charges will accumulate both in thesemiconductor and/or the metal electrode in order to neutralize theferro-electric polarization charges (see FIG. 1 c). These accumulatedcharges induce a strong bending of the energy bands close to theferro-electric semiconductor interface (within a few nm), leading to anenhanced charge injection.

It is not obvious whether the ferro-electric polarization can alsoswitch an Ohmic contact off. In that case, charges of the opposite signneed to be accumulated, which can not be easily injected from thecontact. First experiments with Ohmic gold contacts indeed showed thatthe injection can also be reduced by poling in the opposite direction.

The conductance switching, due to the ferroelectric switching of thedevices, paves the way toward the fabrication of extremely simple, lowcost, and high density non-volatile memory cells by solution processing.To support this proposition, parameters such as area scaling, switchingtime and retention time of the devices need to be considered. FIG. 4shows current scalability for devices with different area. This assuresthat on one hand conduction is due to transport through rir-P3HT, and onthe other hand demonstrates the ability for downscaling the deviceswhich is very important for high density memory applications.

Another crucial parameter to compare with the competing technologies isthe programming time of the devices, that is how fast the polarizationswitches or in the other word the device is turned ON and OFF. In orderto measure the programming time, a pulse of −30V amplitude is appliedacross the device to switch it into the OFF state. Then a pulse of thesame amplitude and opposite polarity with a certain width in time isapplied to switch the device back to the ON state. After this procedurethe current is measured. This cycle is repeated several times, each timeusing a different width for the positive pulse to retrieve an on/offratio larger than 10. FIG. 5 shows the programming time of the device.For a pulse width of 0.5 ms the on/off ratio of 10 is retrieved. Weconclude therefore that the programming time of our devices amounts totrypically ˜0.5 ms.

In FIG. 6 the retention time of the device is demonstrated. The observedlong retention time is due to the polarization stability of theferroelectric. Therefore, as long as the ferroelectric retains itspolarization, which is at least a few years for P(VDF-TrFE), the devicewill be able to maintain its on/off ratio as long as the semiconductor(in our case rir-P3HT) does not degrade. Retention measurements arecurrently carried out on two different devices; one is held in theon-state and the other in the off-state. Both of them did not show anysign of degradation after four months.

An important issue is whether the concept of a modulatable injectionbarrier is universal and valid for a large number of ferro-electricsemiconductor combinations. In FIG. 7 the IV characteristics are shownfor a device in which the polymeric semiconductor rir-P3HT is replacedby another organic semiconductor, the molecule Ni-dithielene. The blendconsists of P(VDF-TrFE) 65%-35% with 5% of the Ni-complex, sandwichedbetween two Ag electrodes. The switching behaviour is identical to therir-P3HT devices and shows that the concept of a modulatable injectionbarrier is not limited to only one specific materials combination.

For arrays of memory devices it is important to avoid cross-talk betweenthe memory elements. The way to circumvent cross-talk is to make a diodeof which the resistance is only modulated in forward bias. In reversebias the diode should always be closed. In FIG. 8 the I-Vcharacteristics of a blend 1:40 rir-P3HT:P(VDF-TrFE) with asymmetricelectrodes are shown. As a bottom electrode Au is used, which is a goodhole injecting contact. As a top electrode the low work function LiF/Alis used, which is a very poor hole injector. Upon poling the injectionof holes from the Au into rir-P3HT is modified (negative bias), whereasthe injection of holes from the LiF/Al is always blocked, independent ofthe poling direction. This example shows that it is possible tofabricate a programmable rectifying diode of which the current is onlymodulated in forward bias, in reverse the current is always off.

A large disadvantage of organic light-emitting diodes is that a reactivemetal is required to efficiently inject electrons. We have demonstratedthat efficient electron injection can also be achieved from anintrinsically bad injecting contact. This was realized using aferro-electric:semiconductor MEH-PPV blend together with an Al electrodemodify the electron injection.

1-16. (canceled)
 17. A use, in a semiconductor element, of a blend of asemiconductor material and a ferro-electric material to provide amodulatable injection barrier, said modulatable injection barrier beingan energy barrier that electrons or holes overcome when injected from anelectrode into a semiconductor material, at the interface of theelectrode and the semiconductor material, said modulatable injectionbarrier being formed between at least one electrode layer and asemiconductor layer, wherein the semiconductor layer comprises saidblend, and modulation of the modulatable injection barrier beingperformed by modulating poling of the ferro-electric material.
 18. Theuse according to claim 17, wherein the ferro-electric material and thesemiconductor material are organic materials.
 19. The use according toclaim 18, wherein the ferro-electric material is a nylon.
 20. The useaccording to claim 18, wherein the semiconductor material is: an organicmaterial selected from the group consisting of: fullerenes, pyrilenes,phthalocyanines, oligomers of thiophenes, phenylenes and phenylenesvinylenes; or a polymeric material selected from the group consisting ofpoly(3-alkylthiophene)s, poly(dialkoxy phenylene vinylene)s,poly(aniline)s, poly(thiophenes), poly(phenylene)s, poly(phenyleneethylene)s, poly(pyrole)s, poly(furna)s, poly(acetylene)s,poly(arylenmethine)s, poly(isothianaphthene)s, poly(fluoren)s, andregion-irregular poly 3-hexyl-thiophene rir-P3HT.
 21. The use accordingto claim 18, wherein the blend of the semiconductor material and theferro-electric material comprises a block copolymer having asemiconducting block and a ferro-electric block.
 22. The use accordingto claim 18, wherein the blend of the semiconductor material and theferro-electric material comprises an interpenetrating polymer networkwith separate polymer phases.
 23. A semiconductor element comprising atleast one modulatable injection barrier, said barrier being formedbetween at least one electrode layer and a semiconductor layer, whereinthe semiconductor layer comprises a blend of a semiconductor polymer anda ferro-electric polymer, the blend comprising the ferro-electricpolymer in at least a sufficient quantity to allow that a polarizationcharge can be measured, and the semiconductor polymer in an amount atleast sufficient to allow a path through the blend for travel of acharge carrier between electrodes.
 24. The element according to claim23, wherein the blend comprises the ferro-electric polymer and thesemiconductor polymer in a ratio, by weight, ranging from 1:1 to 1000:1.25. A device comprising at least one semiconductor element comprising atleast one modulatable injection barrier, said modulatable injectionbarrier being formed between at least one electrode layer and asemiconductor layer, wherein the semiconductor layer comprises a blendof a semiconductor polymer and a ferro-electric polymer, the blendcomprising the ferro-electric polymer in at least a sufficient quantityto allow that a polarization charge can be measured, and thesemiconductor polymer in an amount at least sufficient to allow a paththrough the blend for travel of a charge carrier between electrodes. 26.The device according to claim 25, in the form of a three-terminal devicecomprising the blend as an active layer on top of an insulating layer,wherein the insulating layer is sandwiched between a gate electrode andthe active layer, and wherein a source electrode and a drain electrodeare applied to the active layer.
 27. The device according to claim 26,being a two-terminal non-volatile programmable resistor comprising: (a)a first electrode layer; (b) a second electrode layer; and, in contactwith said electrode layers, and (c) a storage layer comprising aferro-electric polymer, which storage layer separates the first andsecond electrode layers from each other, wherein the storage layercomprises a blend of the ferro-electric polymer and a semiconductorpolymer.
 28. The device according to claim 25, wherein the electrodesare made of metals that do not form Ohmic contacts with thesemiconductor material.
 29. The device according to claim 25, built upso as to have crossing bars of electrodes.
 30. The use of a deviceaccording to claim 25, wherein the modulatable injection barrier servesto program a memory device, to select a switch position, or to reverse adiode.
 31. The use of a device according to claim 25, wherein themodulatable injection barrier serves to switch on or off the injectionof one type of charge carrier (electrons or holes) into a light-emittingsemiconducting layer.
 32. A rectifying organic or polymeric memorydevice comprising a modulatable injection barrier, said modulatableinjection barrier being formed between at least one electrode layer anda semiconductor layer, wherein the semiconductor layer comprises a blendof a semiconductor polymer and a ferro-electric polymer, the blendcomprising the ferro-electric polymer in at least a sufficient quantityto allow that a polarization charge can be measured, and thesemiconductor polymer in an amount at least sufficient to allow a paththrough the blend for travel of a charge carrier between electrodes 33.The use according to claim 18, wherein the ferro-electric material is apolyvinylidene fluoride copolymer with poly trifluoroethylene.
 34. Anelement according to claim 23, wherein the blend comprises theferro-electric polymer and the semiconductor polymer in a ratio, byweight, ranging from 10:1 to 100:1.